A cell is a basic unit constituting a battery also an electrical energy storage device. Among others, lithium-ion batteries are the currently most popular battery devices. However, the lithium-ion battery is not a very stable device and there are risks and limits in using it. For example, the lithium-ion battery in use can not be overcharged or deeply discharged; otherwise the battery would become damaged or unusable. Further, the lithium-ion battery should not be charged at an environmental temperature higher than the temperature range preset for it. The lithium-ion battery has different discharge curves, discharge voltages and discharge time when being discharged at different temperatures.
While a capacitor has the property of storing electrical energy temporarily, it could not be used as an electrical energy storage device. FIG. 1A shows a non-polarized capacitor, which is usually used in an electrical coupling circuit; FIG. 1B shows a polarized capacitor, which is usually used in circuits for filtering and buffering; FIG. 1C shows a polarized supercapacitor, which is usually used in circuits for temporary electrical energy storage, but it is not really an electrical energy storage device; FIG. 1D shows a non-polarized supercapacitor, which can be polarized in circuits and is usually used in circuits for temporary electrical energy storage, but it is not really an electrical energy storage device; and FIG. 1E shows a polarized pseudocapacitor, which is usually used in circuits as a filter for large direct-current (DC) transmission. None of the above five types of capacitors can be really used as a device for storing electrical energy and they do not provide the function of a battery or a rechargeable battery.
The lithium-ion battery is characterized by its high specific energy, which also results in its high internal resistance, or high direct-current resistance (DCR), and is therefore not suitable for fast charge and fast discharge. Some people connect the lithium-ion battery and the pseudocapacitor in parallel for use. The pseudocapacitor can have a large capacity nearly as high as a battery's. The pseudocapacitor has the property of isolating DC, which causes a certain degree of difficulty in charging it. When the lithium-ion battery and the pseudocapacitor are connected in parallel for use, the circuit so formed is subject to the risk of instantaneous short circuit when being charged by high voltage and therefore has poor operational stability.
There are also people who connect the lithium-ion battery and the supercapacitor in parallel for use. The supercapacitor tends to produce a strong electrostatic field, i.e. a polarization effect, when it is being charged. Further, as a property of the capacitor, the charging of the capacitor is impeded by its capacitive reactance to thereby cause rising potential and temperature thereof, creating an environmental temperature that is disadvantageous to the lithium-ion battery. That is, neither the parallelly connected lithium-ion battery and pseudocapacitor nor the parallelly connected lithium-ion battery and supercapacitor could eliminate the problem of poor operational stability of the lithium-ion battery.